Electrically heated personal comfort or medical aid devices typically include an electrical resistance heater wire threaded between a pair of fabric covers. Heat is generated and supplied to the user when electrical energy is applied across the heater wire. To provide a uniform distribution of heat, the heater wire is usually distributed or located in a plurality of parallel, evenly spaced channels that extend across substantially the entire area of the blanket or other device. Although reference will be made to an electric blanket throughout, it is to be understood that the invention may be used with other personal heating items, such as hot pads, heated socks, etc., and with other items where heating is sought with protection against overheating.
To guard against overheating a control is included for the user to manually open the circuit providing electrical energy to the heater wire. Overheating may result from the placement of a thermally insulating cover over the blanket, a bunching of the blanket, irregularities in the amount of electrical energy applied to the heater wire, or an electrical or mechanical malfunction. Such an overheating may be detrimental to the user and materials adjacent the heater wire, so it is also desirable to have a self monitoring heat sensitive device that will automatically control the flow of electrical energy to the heater wire in the event that the user does not detect an overheating condition or fails to manually open the circuit.
In the past, a variety of overheating protection circuits have been utilized, but all had drawbacks. In one arrangement, a number of conventional bimetallic thermostatic switches were placed in series with the heater wire at various locations throughout the blanket. These switches were normally closed, but when an overheat temperature was sensed in the vicinity of a thermostatic switch, it opened, interrupting the flow of electrical energy and the generation of heat. This approach suffered due to the bulk and expense of the thermostatic switches, and due to the limited area or number of zones that were monitored by each switch, which left certain portions of the blanket unmonitored.
A type of thermostatless overheat protection circuit was disclosed in commonly assigned U.S. Pat. No. 4,198,562 issued Apr. 15, 1980. It employed a pair of solid state switching devices in the heating element circuit. The gates thereof were electrically connected in series by a single conductor electrically connecting a finite number of discrete sensors or thermistors. They were connected in series and rendered the switching devices nonconductive if an overheating condition was sensed.
Another type of thermostatless overheat protection circuit was disclosed in commonly assigned U.S. Pat. No. 3,683,151 issued Aug. 8, 1972. This circuit included a sensor having a temperature sensitive dielectric between a pair of signal wires. A single gate controlled semiconductor switch responded to a change in the dielectric characteristic of the material between the sensor wires to reduce the energy delivered to a heating element in response to an overheating condition. Commonly assigned U.S. Pat. No. 4,315,141 issued Feb. 9, 1982 disclosed a pair of solid state switching devices in a heating element circuit. The gates of each semiconductor switch were electrically connected to dual conductor temperature sensors having a temperature sensitive dielectric between them. The switching devices were rendered nonconductive if an overheating condition was sensed. These circuits utilized relatively expensive sensors, and included additional wires not found in the present invention, which may reduce their operational lifetime.
Other patents, such as U.S. Pat. No. 3,410.984 issued Nov. 12, 1968, disclose electrically heated bedcovers with a self regulating heater formed by a pair of resistance conductors separated by a layer of material having a positive temperature coefficient (PTC) of resistance. As the temperature of the PTC material increases, the impedance increases, thereby reducing current flow between the separated conductors, and reducing heat output. It does not include discrete heating means and an electrical sensor, nor does it include a control means as claimed herein.
Thus, it is an object of the present invention to provide an improved electrical heating apparatus which is protected against overheating and which is less subject to any of the aforementioned deficiencies.
It is a further object of the present invention to provide an improved electrical heating apparatus protected against an overheating condition which monitors all heated portions of a given heating apparatus rather than a limited number of discrete zones, and utilizes a heat sensitive conductor consisting of a single conductor.
It is still a further object of the present invention to provide a comparatively inexpensive electrical heating means and heat sensitive conductor consisting of a single conductor that may be encased in a jacket to form a single strand for placement between a pair of fabric covers.